A solid-white paint composition which has a high concealing capacity in which a finely divided flake-like pigment is compounded to provide the concealing properties in a white paint composition which includes titanium dioxide is known as a solid-white paint composition which has a high concealing capacity which provides satisfactory concealing properties when coated directly by means of electro-deposition coating without the application of a mid-coat paint. Furthermore, a method of coating which is characterized in that coating is carried using the solid-white paint composition which has a high concealing capacity on an under-coat coated base material without forming a mid-coat paint film and cured without forming a clear paint film over the top has also been disclosed (for example, see Japanese Unexamined Patent Application laid open H11-049993). However, when a fine aluminum pigment or a fine graphite pigment is used for the finely-divided flake-like pigment in order to obtain good concealing properties, the color as a metallic pigment of black pigment appears strongly and there is a disadvantage in that the paint film does not have a solid-white color. In addition, the paint film thickness when curing must be from 10 to 60 μm, and preferably from 15 to 40 μm (from 35 to 65 μm in the illustrative examples) and this is disadvantageous in terms of cost.
Furthermore, a method comprising the sequential over-coating of an aqueous white based paint (A), an aqueous mica-based paint (B) and an organic solvent type acid/epoxy based clear paint (C) on the object which is to be coated is known as a method for the formation of a multi-layer paint film for automobile purposes, and it has been disclosed that the whiteness of the paint film alone as an L-value is ideally at least 85 and more desirably at least 90, and that a white pigment which has a particle diameter generally within the range from 0.0001 to 0.5 μm, such as titanium white, flowers of zinc or the like, is compounded in an amount generally within the range from 50 to 200 parts by weight per 100 parts by weight in total of the base resin and crosslinking agent as the white pigment which can be compounded in the white base paint (for example, see Japanese Unexamined Patent Application laid open 2003-334488). However, with this method the object to be coated is an object where a mid-coat paint has been coated on an electro-deposited paint film and so the concealing properties of the base paint are inadequate when no mid-coat paint has been applied.
Furthermore, a method of coating which is characterized in that, in the coating process of an aqueous top coat base paint and clear paint the aqueous paint coating process which has a first aqueous paint coating process in which the inner panel parts of the aforementioned automobile body are coated and then a second aqueous paint coating process in which the outer panel parts of the aforementioned automobile body are coated, the set temperature of the air in the aforementioned first flash-off process is set in such a way that the painted solid fraction of the paint film on the inner panel parts of the automobile body coated in the aforementioned first aqueous paint coating process is above a prescribed value when the aforementioned second flash-off process is proceeding and is completed is known (for example, see Japanese Unexamined Patent Application laid open 2005-177632). Thus, it is disclosed that the set temperature of the first flash-off process is set so that the deposited solid fraction of the paint film on the inner panel parts of the body which have been coated in the inner-panel coating zone is at least 70 wt % when the second flash-off process is passed through and completed. However, with this method the relationship between deposited solid fraction and the paint solid fraction immediately before coating is unclear and no findings in connection with the relationship between the volume shrinkage of the aqueous paint and the concealing properties have been obtained.
The present invention is intended to provide a method whereby very brightly colored, and especially white or lightly colored, multi-layer paint films which have excellent concealing properties are formed in a method for the formation of multi-layer paint films in which aqueous colored paints in which a specified titanium oxide has been compounded in a prescribed amount are used and the coating process with a mid-coat paint is omitted.
As a result of thorough research carried out with a view to resolving this problem, the inventors have discovered that the abovementioned problem can be resolved with a multi-layer paint film where aqueous paint in which a prescribed amount of a specified titanium oxide has been compounded is used over a cured paint film of an electro-deposition paint, and the invention is based upon this discovery.